This invention relates to an image forming s, and more particularly to an image forming apparatus having a counter for indicating a time for exchange of a filter used in a heat-fixing process.
There has been well known a conventional image forming apparatus such as a copying machine in which a photosensitive and pressure-sensitive recording medium coated with microcapsules encapsulating dye precursor therein is exposed to light to form a latent image thereon, are used. The recording medium is superposed over a developer sheet coated with developer material color-reactable with the dye precursor under pressure to pressure-develop the latent image on the photosensitive and pressure-sensitive recording medium into a visible image on the developer sheet. In this type of image forming apparatus, after the above pressure-developing process, the developer sheet having the visible image thereon is fed to heat-fixing unit comprising a heater where it is a fan, and heated by a heated air or the like to thereby heat-fix the visible image on the developer sheet (that is, perform a heat-fixing process).
In this type of heat-fixing unit, foul and harmful gas is emitted from the developer material coated on a base sheet of the developer sheet due to heat during the heat-fixing process. Thus a user must carry out a copying operation under a harmful condition. Accordingly, it has been required in the art to withdraw the gas including iou and harmful components and protect the user against the foul and harmful components during a series of image forming processes of the apparatus. In order to satisfy this requirement, the heat-fixing unit is equipped with a gas purifying unit having a suction fan for sucking the gas therethrough and a filter for trapping only the foul and harmful components of the gas and discharging only clean air to an outside of the apparatus. The filter is gradually degraded and finally has no capability of trapping the components after a long use thereof. Accordingly, in order to indicate a time for exchanging a used filter for a new one, the apparatus is further provided with a counter (or counting the number of developer sheets which have been subjected to the heat-fixing process (that is, the number of times of the heat-fixing process has been carried out since a filter was exchanged). In this case, when a value counted by the counter reaches a predetermined number, the used filter is exchanged for a new filter.
In the conventional image forming apparatus having the gas purifying unit and the counter, all types of developer sheets are counted equally by the counter irrespective of the kind of the developer sheets that is subjected to a heat-fixing process. For example, when a heat-fixing process is conducted on an OHP developer sheet comprising a base sheet of transparent resin film such as polyethylene film with the developer material coated on the sheet, a larger amount of gas is emitted from the base sheet and the developer material than when a heat-fixing process is used with another type of developer sheet comprising a base sheet of plain paper with developer material coated on the sheet. Accordingly, for use of the OHP developer sheets, the filter is degraded in gas purifying capability before a counted value of the counter reaches the predetermined number, so that foul and harmful gas is not trapped in the filter and thus is discharged charged from the apparatus before a time for exchange of the filter is indicated.